Chlorinated hydrocarbon compounds, in particular, chlorinated hydrocarbon solvents, are prevalent contaminants of groundwater and aquatic sediments. Halogenated hydrocarbons have low flammability and are fairly stable, both chemically and biologically. They have been commonly used in industry as chemical carriers and solvents, paint removers, and cleaners. Chlorinated solvents are stable compounds that are relatively toxic at low levels, and many chlorinated solvents have been classified as suspected or confirmed carcinogens. Chlorinated hydrocarbon solvent contaminants include chlorinated ethylene compounds such as tetrachloroethylene (also known as perchloroethylene, PCE), trichloroethylene (TCE) and dichloroethylene (DCE), as well as various other halogenated aliphatic compounds and solvents.
In the United States, both PCE and TCE were found at 852 of 1430 National Priority List (NPL) sites in 1997, establishing these chlorinated hydrocarbon compounds as two of the most common groundwater contaminants identified in superfund sites (Doherty, R. E. 2000. A history of the production and use of carbon tetrachloride, tetrachloroethylene, trichloroethylene and 1,1,1-trichloroethane in the United States: Part 2-Trichloroethylene and 1,1,1-trichloroethane. J. Environ. Foren. 1: 83-93). These contaminants typically originated from residual dumps from industrial use, or from spills or leaks. Typical contaminated sites contain chlorinated hydrocarbon solvents dissolved in groundwater, chlorinated hydrocarbon solvents in ground dense non-aqueous phase liquid (DNAPL), or both. Even a small spill can result in large, dispersed plumes of chlorinated hydrocarbon solvents that are very difficult and expensive to treat. Thus, even relatively small amounts of solvent can pose serious risks to the environment and to water supplies.
In situ remediation of contaminated groundwater aquifers allows treatment of contaminants on site, which can lead to less expensive and less destructive processes of removal. However, current abiotic and biotic in situ remediation strategies available for groundwater contaminated with chlorinated DNAPLs have limitations and drawbacks.
For example, abiotic treatment methods have included passive systems, such as permeable reactive barriers that can be constructed of materials like zero-valent iron. This method can be costly, depending on the site location and depth of the contaminated aquifer. Electrochemical reactors have also been investigated as a means for abiotic reductive dehalogenation of chlorinated groundwater contaminants. Such abiotic processes are limited by electrode material, temperature, ionic strength and pH of the groundwater, transportation of the target compound to the surface of the cathode, and the rate of charge transfer to the surface of the cathode.
As an alternative to abiotic groundwater treatment, naturally occurring biotic reductive dechlorination has been applied to field site applications for removal of chlorinated DNAPLs. Bioremediation of chlorinated hydrocarbons by microorganisms that respire compounds via reductive dechlorination has been studied and implemented as a biological treatment of compounds such as PCE and TCE through bioaugmentation, the addition of dechlorinating microorganisms, or biostimulation, the stimulation of existing dechlorinators through the addition of electron donors or nutrient amendments (Czaplicka, M. 2004. Sources and transformations of chlorophenols in the natural environment, Sci. Total Environ. 322:21-39). Limitations of current applications of bioremediation for contaminant removal from groundwater include buildup of toxic degradation products, such as vinyl chloride (VC). Additionally, when hydrogen is supplied as an electron donor for stimulation of bioremediation, dechlorinating microbes compete with other hydrogen-utilizing bacteria such as nitrate and sulfate reducers, methanogens, and acetogens. Yet another limitation of reductive dechlorination in a source zone contaminated with chlorinated DNAPLs is intolerance of dechlorinating microorganisms to saturated concentrations of chlorinated substrate, commonly found at source zones.
The possibility of promoting degradation of chlorinated contaminants with electrical current has been investigated previously. However, these approaches are not appropriate for in situ treatment of chlorinated contaminants in aquifers or in aquatic sediments. For example, electrodes poised at low potentials can abiotically dechlorinate the chlorinated solvents PCE and TCE as well as less chlorinated compounds such as VC and the dichloroethylene isomers (DCE). Potentiostat-poised electrodes used to supply hydrogen as an electron donor to bacteria have been suggested for use in groundwater or soil to stimulate reductive dechlorination. Additionally, electrolysis of chlorinated compounds has been suggested to inadvertently stimulate soil microbes (Skadberg, B., et al., 1999. Influence of pH, current and copper on the biological dechlorination of 2,6-dichlorophenol in an electrochemical cell. War Res. 33(9): 1997-2010). However, these approaches have significant limitations for in situ treatment of groundwater or soil because: 1) hydrogen gas is produced, which can stimulate the growth of anaerobic microorganisms resulting in the accumulation of unwanted biomass and production of undesirable end-products, such as Fe(II), sulfide, and methane that deteriorate water quality; 2) reduction of protons results in high groundwater pH which disrupts biological, chemical, and physical properties of the soil; and 3) non-specific reduction of protons, and possibly other groundwater constituents, results in poor efficiency in electron transfer to the chlorinated contaminants of interest.
Transferring electrons from an electrode to microorganisms capable of reductive dechlorination has been proposed through use of shuttle compounds, such as methylviologen (Aulenta, F., et al., 2007. Electron transfer from a solid-state electrode assisted by methyl viologen sustains efficient microbial reductive dechlorination of TCE, Environ. Sci. Technol. 41: 2554-2559). However, methylviologen (1,1′-dimethyl-4,4′-bipyridinium dichloride, also known as paraquat dichloride) itself is highly toxic and thus is considered a contaminant.
Certain microorganisms have been shown to be electricigenic, and interact electrochemically with electrodes without requiring an electron shuttle such as methylviologen. As used herein, “electricigen” or “electricigenic bacteria” refer to microbes that conserve energy to support growth by completely oxidizing organic compounds to carbon dioxide with direct electron transfer to the anodes of microbial fuel cells. Electricity production with electricigens is significantly different from that of other types of microorganisms. See, e.g., Lovley, D. R & Nevin, K. P. (2008) Electricity Production with Electricigens, pp. 295-306 in Wall, J., et al., ed. Bioenergy, ASM Press, Washington, D.C. Electricigens have the ability to oxidize organic compounds to carbon dioxide with an electrode serving as the sole electron acceptor, providing high coulombic efficiency that is not available with other microbes.
In such cases, the electricigenic microorganism was found to donate electrons to an anode of a microbial fuel cell, which serves as an electron acceptor for these microorganisms. See Bond, D. R., D. R., Lovley. 2003. Electricity production by Geobacter sulfurreducens attached to electrodes. Appl. Environ. Microbiol. 69(3): 1548-1555; Bond, D. R., et al. 2002. Electrode-reducing microorganisms that harvest energy from marine sediments. Science. 295: 483-485. There have also been limited demonstrations where an electrode poised at a sufficiently low potential has been shown to serve as an electron donor for Geobacter sulfurreducens or Geobacter metallireducens. These species were shown to accept electrons from a graphite electrode for the reduction of more electrochemically positive electron acceptors such fumarate, nitrate, or U(VI). See Gregory, K. B., et al., 2004. Graphite Electrodes as electron donors for anaerobic respiration. Environmental Microbiology 6(6): 596-604; and Gregory, K. B., and D. R. Lovley. 2005. Remediation and recovery of uranium from contaminated subsurface environments with electrodes, Environ. Sci. Technol. 39: 8943-8947.
For reductive dechlorination as a bioremediation strategy to be successful, dechlorinating microorganisms and a suitable electron donor must both be localized at or near a source zone of contamination, and be effective without producing resulting toxic effects. Appropriate addition of a chemical electron donor without unduly stimulating growth of unwanted non-dechlorinating microorganisms, or inhibiting growth of beneficial dechlorinating microorganisms can be particularly challenging.